Because the calculation works best for estimating reduced kidney function, the National Kidney Foundation suggests only reporting actual results once values are < 60 mL/min (they state normal values as 90-120 mL/min). An eGFR below 60 mL/min suggests that some kidney damage has occurred.
So 60 is the number to watch for.
We had someone in our group end up on dialysis after Harvoni BUT he started with a GFR below 60. He probably would have ended on dialysis anyway.
P